This invention relates to catalysts which can convert sulfur dioxide to elemental sulfur, and the preparation method needed for their manufacture.
Industries Generating SO.sub.2
A chronic concern for the environment has been release of pollution from industrial and other sources into the air and water. Of particular concern to environmentalist are gaseous emission containing sulfur dioxide. When this gas rises to cloud level, the rain produced from these clouds can become highly acidic, at times reaching the acid levels of vinegar. Because the emissions and clouds effected by them can travel great distances beyond the point of initial emission, this form of pollution takes on international dimensions.
The effects of such industrially related acid rains are infamous. Streams and lakes in North Eastern America and Canada have been rendered devoid of their natural flora and fauna due to acidification by acid rain. Trees in these areas have also been baldly compromised. Similar effects have been seen in Europe, where large section of trees in the famous Black Forest have been damaged, and in some cases destroyed, by the effects of acid rain.
Flue gases emitted from burning sulfur-containing fossil fuels are the most common source of dilute sulfur dioxide (So.sub.2) containing industrial gases. Sulfur dioxide is the unwanted byproduct of a diverse array of industrial activities, such as coal-burning power plants, advanced integrated gasification combined cycle (IGCC) using hot gas clean up systems, petroleum sulfur plants tail gas treatment facilities, metallurgical operations, and the like. Because of regulatory constraints, these operations are currently hampered by a limited choice of available coal sources, working parameters, etc., in order to meet environmental regulatory mandates. Further reduction of SO.sub.2 contaminates would allow longer operating times for these facilities while staying within the legally determined limits for emissions.
Regenerable flue gas desulfurization systems (FGD) and integrated gasification combined cycle (IGCC) hot gas cleanup systems have been developed for use in coal-based utility industries. These systems release a stream of highly concentration (1%-30%) SO.sub.2 gas. It is desirable to be able to convert the SO.sub.2 gas to elemental sulfur using a single step process.
The petrochemical industry commonly uses the conventional Claus process for conversion of H.sub.2 S to elemental sulfur. This system releases a stream of tail gas containing SO.sub.2 and H.sub.2 S. An additional tail gas treatment system is then required to reduce these residual gases to below 250 ppm in order to comply with environmental air regulations. Currently available tail gas treatment systems requiring an organic solvent absorption step are complex and expensive. The development of a new tail gas treatment process capable of reducing SO.sub.2 by H.sub.2 /CO to elemental sulfur would be very desirable and cost effective.
Conversion of SO.sub.2 to Elemental Sulfur
Research efforts have been made to allow the conversion of sulfur dioxide to elemental sulfur. In these methods, sulfur dioxide is reduced with synthesis or natural gases. Synthesis gases are derived from coal (H.sub.2 /CO.dbd.0.3-2.0) or methane (H.sub.2 /CO.dbd.3-5). At elevated temperatures, sulfur dioxide can be converted to elemental sulfur through the following reactions: EQU 0.875 SO.sub.2 +0.75 H.sub.2 +CO.fwdarw.0.4375 S.sub.2 +CO.sub.2 +0.75 H.sub.2 0 EQU 2 SO.sub.2 +3H.sub.2 +CO.fwdarw.S.sub.2 +CO.sub.2 +3H.sub.2 O
Sulfur dioxide can be reduced with natural gas (mainly methane) as follows: EQU 2 SO.sub.2 +CH.sub.4.fwdarw.S.sub.2 +CO.sub.2 +2H.sub.2 O
The reactions must be facilitated with catalysts in order to achieve a real time high conversion efficiency of SO.sub.2. Even with the assistance of numerous catalysts, commercially feasible conversion efficiencies have not been achieved.
In addition to elemental sulfur, the above reactions produce a number of undesirable byproducts. These can include hydrogen sulfide, carbonyl sulfide, carbon disulfide, and elemental carbon. These byproducts complicate the ability of the conversion reactions to effectively reduce the net airborne contaminates produced during industrial processing.
Because of the inadequacies of the above reactions when direct to industrial applications, research efforts have been carried out to bring this potentially useful area of technology to a level where it has practical applications. The thrust of these research efforts have been to improve the conversion efficiency of sulfur dioxide and increase the selectivity to the production of elemental sulfur at relatively low temperatures.
While there has been some success in this area of research, the results which have been reported to date can not practically be applied to commercial uses.
Oxide and Metal Catalysts
Responding to the problems of current SO.sub.2 capture methods, the catalyst research community has been attempting to develop regenerable flue gas desulfurization catalysts and processes. Most of the efforts to develop a practical SO.sub.2 reductant catalyst have not progressed beyond basic research. However, recently certain researchers have reported more success.
Oxide and simple metal forms of metals represent the few SO.sub.2 catalysts with appreciable activity. Many challenges have been encountered in this area of research, such as low yields, substantial reduction of sulfur yields by very low levels of water vapor, and unacceptable levels of unwanted byproducts.
By using syngas to reduce SO.sub.2, Akhmedov et. al. (Azerb Khim. Zh(2), 95-9, 1983) developed several catalysts and obtained the following results: a 64-65% sulfur yield with a bauxite-bentonite catalyst at 350.degree. C. with a feed gas at a molar ratio (CO+H.sub.2)/SO.sub.2 of 2 and a space velocity of 1000 h.sup.-1 ; a 82% sulfur yield with a NiO/Al.sub.2 O.sub.3 catalyst at 300.degree. C. with a space velocity of 500 h.sup.-1 (Zh. Prikl., Khim., 61(1), 16-20, 1988); an 82% and 87.4% sulfur yield with a Co.sub.3 O.sub.4 /Al.sub.2 O.sub.3 catalyst (Zh. Prikl. Khim., 61(8), 1891-4, 1988) at 300.degree. C. with a space velocity of 1000 h.sup.-1 and 500 h.sup.-1 respectively; a 82.3% and 78.6% sulfur yield with a NiO+Co.sub.3 O.sub.4 catalyst (Khim. Prom. 1, 37-9, 1989) at 400.degree. C. with a space velocity of 500 h.sup.-1 and 1000 h.sup.-1 respectively. The development of a catalyst capable of obtaining better than 90% yield of sulfur with a high space velocity at low temperatures would be required to warrant a commercial application.
Natural gas or methane can also be used as reducing gases to recover elemental sulfur from SO.sub.2. However, this process requires elevated temperatures, and typically produces undesirable byproducts such as hydrogen sulfide, carbonyl sulfide, carbon monoxide, and elemental carbon. A plant capable of producing 5 tons per day of elemental sulfur through the reduction of SO.sub.2 by natural gas was developed and in operation in 1940 (Fleming et al, Industrial Enaineering Chemistry Vol. 42, p2249, 1950). Because a secondary reactor was required to treat byproducts, this process was economically unattractive.
A more efficient process requiring two stages was developed to avoid some of the problems of the Fleming system. In the first step, part of the SO.sub.2 was reduced to H.sub.2 S by methane and/or low value hydrocarbons. In the second step, the H.sub.2 S and the remaining SO.sub.2 were converted to elemental sulfur in a multi-stage Claus unit (Bierbowere, et al Chemical Engineering Progress August, 1974).
Numerous research efforts have been made to develop a catalyst for sulfur dioxide reduction where the amount of byproducts produced is negligible, so that a second stage treatment will not be required. To date, none have been successful enough to warrant application to an industrial facility.
Flytzani-Stephanopoulos et al reported favorable results with mixed oxide catalysts in a limited environment, but rapidly decreasing sulfur yields in the presence of only one or two percent of water in the feed stream. In U.S. Pat. No. 5,242,673 (issued Sep. 7, 1993) Flytzani-Stephanopoulos et al taught a cerium oxide catalyst which, in a dry environment with CO in stolchiometeric amounts had better than 90% conversion rate (column 8, line 53-55), and in another case, a selectivity of sulfur dioxide toward elemental sulfur of 50-60% (column 9, lines 15-17).
In U.S. Pat. No. 5,384,301, (issued Jan. 24, 1995) Flytzani-Stephanopoulos et al teach several new sulfur dioxide reducing oxide catalysts. CeO.sub.2 (La) showed a 95% conversion of SO.sub.2 to elemental sulfur in a CO gas stream. With the addition of transition metals to produce Cu/CeI.sub.2 M, Cr/CeO.sub.2, and Ni/CeO.sub.2, the same level of conversion to sulfur was obtained. However, with only 2% H.sub.2 O in the gas stream, the conversion to sulfur dropped to 72%. These sulfur yields were also reflected in the Cu.sub.0.15 Ce.sub.0.85 O.sub.1.85, Cu.sub.0.026 Pr.sub.0.035 Ce.sub.0.65 O.sub.1.85, Gd.sub.2 Zr.sub.2 O.sub.7, and Cu.sub.0.15 (Gd.sub.2 Zr.sub.2).sub.0.8506.1 catalysts these researchers reported.
Sulfide Catalysts
Sulfide forms SO.sub.2 reductant catalysts have not been pursued by the catalyst research community beyond some very limited initial work due to the discouraging findings. Khalafalla et al found that when the iron on an iron-alumina catalyst was transformed to FeS, there was a decrease in activity, and in some cases showed no activity at all (Khalafalla et al. Journal of Catalysis vol. 24, pp. 121-129, 1972). Correlation of sulfidation with unwanted byproduct formation has also been reported (Haas et al., Journal of Catalvsis vol. 29, pp 264-269, 1973). The present inventors have also developed sulfur dioxide reducing catalysts, as described in U.S. Pat. No. 5,494,879 issued Feb. 27, 1996 and incorporated by reference herein.
A comparison of the oxide and sulfide forms of SO.sub.2 catalysts is shown in the Comparison Chart below. The poor performance of the sulfide forms have led researchers to pursue the oxide forms of the catalysts.
SINGLE METAL SO.sub.2 CATALYSTS COMPARING YIELDS FOR OXIDE AND SULFIDE FORMS M.sub.2 *O.sub.3 /AL.sub.2 O.sub.3 M.sub.2 S.sub.3 AL.sub.2 O.sub.3 t Y (S.sub.2) t Y (S.sub.2) Cat (.degree. C.) Reductant (%) (.degree. C.) Reductant (%) Cr*/Al.sub.2 O.sub.3 400 CO + H.sub.2 52.0.sup.[2] 370 H.sub.2 24.2.sup.[2] Mo*/Al.sub.2 O.sub.3 450 H.sub.2 62.8.sup.[1] 370 H.sub.2 49.1.sup.[3] Cu*/Al.sub.2 O.sub.3 400 CO + H.sub.2 35.sup.[2]/ 370 H.sub.2 20.1.sup.[3] Co*/Al.sub.2 O.sub.3 400 CO + H.sub.2 70.sup.[2] 370 H.sub.2 60.9.sup.[3] Ni*/Al.sub.2 O.sub.3 450 H.sub.2 61.7.sup.[1] 370 H.sub.2 58.6.sup.[3] *M = Cr, Mo, Cu, Co, N; .sup.[1] Alkhazov et al Zh. Prinkl. Zh. (J. Appl. Chem.) (9) 1826-31 (1991) (Russ) .sup.[2] M.M. Akhmedov et al Khim. Prom. (Chem. Ind.) [1] 37-9 (1989) (Russ) .sup.[3] Paik et al, Appl. Catal. B8 267-79 (1996).
It would be highly desirable to convert the many sources of sulfur dioxide from the many industrial activities that produce it to elemental sulfur. If this conversion could be accomplished in a commercially feasible manner, it would allow reclamation of sulfur and its recycling as a valuable chemical.